Not quite. The building — with its white concrete facade, harsh angles and tinted glass — became a fixture of Hong Kong’s downtown. But it was never added to the city’s register of protected landmarks. Now, with Hong Kong officials under pressure to generate revenue, the nearly 12-acre site, which has been valued at over $5 billion, was put up for sale this month. Supporters of the building are scrambling to save it because whoever buys the land underneath would have every right to tear down the post office. “Some people in Hong Kong might think it’s just a white box,” Charles Lai, an architect in Hong Kong, a Chinese territory, said on a fall afternoon outside the post office, where people were lined up inside to mail packages.